This invention relates generally to downhole tools which are invention relates more particularly, but not by way of limitation, to a wireline tool and method for providing real-time surface readouts of drill stem test data generated during a combined tubing conveyed perforating and drill stem testing operation.
In drilling and operating a well, downhole tools are used to monitor downhole conditions, such as temperature and pressure, to obtain information which is helpful in evaluating the nature of the well, such as whether the well is likely to produce. One particular condition which is preferably monitored is reservoir pressure measured over periods of time during which the well is alternately allowed to flow and prevented from flowing. This condition is determined by means of a drill stem test which can be conducted utilizing the Bourdon tube technique known in the art. With this technique a chart having a pressure versus time graph scribed thereon is obtained.
A shortcoming of the Bourdon tube technique is that no real-time or substantially instantaneous readout of the sensed pressure is available at the surface while the pressure is being detected. A real-time readout is needed to permit a person at the well site to quickly know what is occurring downhole during the test periods. This shortcoming exists because to perform a drill stem test using the Bourdon tube technique, a tool containing an unscribed chart and a bourdon tube instrument are lowered into the well, the well is alternately allowed to flow and prevented from flowing to cause the Bourdon tube instrument to scribe a pressure versus time graph on the chart, and then the tool is withdrawn from the well and the chart analyzed at some relatively considerable time subsequent to the actual time at which the pressures were detected and the chart created.
Electronic, battery powered memory recorders have been developed to store data generated in response to a pressure or other transducer. However, these electronic devices suffer from the same shortcoming as the Bourdon tube technique, i.e., lack of real time readout of data during a test.
Another downhole tool known to me is capable of detecting reservoir pressures, such as during a drill stem test, and of providing real-time surface readouts of the pressure. This prior surface readout instrument includes a valve which is contained within a drill or tubing string located in the well. The valve includes a valve member which is moved downwardly into an open position in response to engagement of the valve member with a housing containing a pressure sensor which is connected by wireline to a surface readout device. Initial movement of the housing into the well is effected by lowering it on the wireline; however, further movement of the housing into engagement with the valve member, and subsequent opening of the valve, is achieved by operation of an electrical, motorized actuator sub of a type known to the art. The actuator sub engages the housing in the well and moves it farther down into the well into engagement with the valve member and on downward until the valve is opened, thereby communicating the reservoir pressure to the pressure sensor.
A tester valve with which this prior surface readout instrument is associated is periodically opened and closed to perform a drill stem test in a manner as known to the art. During the drill stem test, the pressures are detected through the open valve and electrically communicated to the surface via the wireline. When the test has been completed, the actuator sub moves the housing upward in response to electrical commands from the surface. Once the actuator sub has fully disengaged the housing from the valve, the housing and actuator sub assembly are pulled out of the well by reeling in the wireline.
One disadvantage of this prior art surface readout instrument is that it requires electrical power to operate the motor of the actuator sub to engage and disengage the housing (and associated pressure sensor) and the valve member. If the motor fails to operate or if electrical continuity to the motor is lost or if the wireline or cable head develops a short-circuit, for example, the housing and valve member cannot be engaged or disengaged. Such electrical problems are rather frequent because of the extreme downhole environments which are encountered in a well and the relatively long periods of time (days, sometimes) during which the instrument is kept in the well. Another shortcoming of this prior surface readout instrument is that the actuator sub is a complex tool which is difficult to manufacture and difficult to maintain in the field. It is also a relatively expensive tool. Still another shortcoming of the prior art surface readout instrument is that it is relatively long, being almost seventeen feet long in one embodiment of which we are aware. Another type of downhole tool by means of which downhole pressures can be detected and their magnitudes communicated to the surface includes a pressure sensing probe installed in a section of pipe of a pipe string which is to be disposed in the well. This probe is exposed to the borehole environment when the pipe string is in the well, and thus it must be durably constructed to endure the extremes found therein. The magnitude of the pressure detected by this type of probe is communicated to the surface via a connector tool which couples with the probe. The connector tool can be relatively easily removed from the well if a problem occurs; however, if the probe malfunctions or otherwise needs to be removed, the entire pipe string must be removed. This is a significant disadvantage because of the time and expense of tripping the pipe string out of and back into the well.
Therefore, in view of the disadvantages of the aforementioned prior art devices of which we are aware, there was a need for an improved downhole tool and an improved method for using the tool, which tool and method are disclosed in U.S. Pat. No. 4,509,174, issued on Apr. 2, 1985 to Skinner et al, and assigned to the assignee of the present invention.
In particular, this improved tool is able to sense reservoir pressure which is to be monitored during a drill stem test, for example, and to communicate the magnitude of the sensed pressure to the surface for providing a real-time readout of the pressure magnitude.
The aforesaid tool is constructed so that it can be installed and removed with downhole mechanical means, rather than downhole electrical means, to obviate the necessity of an actuator sub and the related electrical circuitry which is subject to the aforementioned problems. To assist in the mechanical manipulation of the tool, there is included means for jarring, or applying force impulses, to the tool to assist in the mechanical coupling and decoupling of the tool elements. The tool also includes a housing for protectively containing a sensor, which housing and sensor can be removed from the well without removing the pipe string in which the tool is to be used, and is constructed to be relatively compact to enhance the transportability of the tool to the well site and the handling of the tool at the well site.
While a great improvement over other prior art devices, the Skinner et al tool has been found to suffer from a deficiency which arises where a drill stem test is conducted in conjunction with a tubing conveyed perforating operation.
In tubing conveyed perforating, which is well known in the art, a "gun" or guns containing a number of shaped charges is lowered into a well bore at the bottom of a string of tubing or drill pipe, and the charges fired to perforate the well bore casing and adjacent producing formation. The advantages of tubing conveyed perforating, as opposed to wireline perforating wherein charges are run on wireline through the tubing or drill pipe, include greater shot density, the ability to perforate in an underbalanced condition (wherein the formation is exposed to a lesser pressure in the tubing string than the hydrostatic present in the annulus) and the ability to perforate virtually unlimited intervals, or lengths, of formation in one trip into the well
To further compound the advantages of tubing conveyed perforating, a technique has been developed wherein drill stem test tools are run into the well above the tubing conveyed perforating gun or guns on the same tubing or drill pipe string, and a drill stem test is conducted immediately subsequent to the firing of the guns rather than at a later time during a subsequent trip into the well.
More specific and detailed descriptions of tubing conveyed perforating alone and in conjunction with drill stem testing may be found, respectively, in U.S. Pat. Nos. 3,706,344 and 4,480,690, assigned to the assignee of the present invention.
It has been found, when conducting a combined tubing conveyed perforating/drill stem testing operating using the real-time surface readout apparatus described in the aforesaid Skinner et al patent, that the connector mechanism by which the probe portion of the apparatus (run on wireline) is secured to the pipe portion of the apparatus (run into the well as part of the tool string) disconnects itself in some instances when hit by the shock wave generated by the firing of the perforating guns. This is due to the nature of the design of the connector mechanism, which connects and disconnects the probe to the pipe string portion in response to reciprocation of the probe induced by manipulation of the wireline from which the probe is suspended. Since it is generally part of such an operation to let the perforated formation flow immediately up the interior of the test string, disconnection of the probe results in the loss of valuable data as well as the possibility of the probe being carried up the string by the formation flow and tangling in or damaging the wireline, the probe itself, or the connections between the two.